RE: [PATCH v4 1/1] drm/i915: Move abs_diff() to math.h

[David Laight]

From: Andrew Morton
> Sent: 03 August 2023 18:25
> 
> On Thu,  3 Aug 2023 16:19:18 +0300 Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> 
> > abs_diff() belongs to math.h. Move it there.
> > This will allow others to use it.
> >
> > ...
> >
> > --- a/include/linux/math.h
> > +++ b/include/linux/math.h
> > @@ -155,6 +155,13 @@ __STRUCT_FRACT(u32)
> >  	__builtin_types_compatible_p(typeof(x), unsigned type),		\
> >  	({ signed type __x = (x); __x < 0 ? -__x : __x; }), other)
> >
> > +#define abs_diff(a, b) ({			\
> > +	typeof(a) __a = (a);			\
> > +	typeof(b) __b = (b);			\
> > +	(void)(&__a == &__b);			\
> > +	__a > __b ? (__a - __b) : (__b - __a);	\
> > +})
> 
> Can we document it please?
> 
> Also, the open-coded type comparison could be replaced with __typecheck()?
> 
> And why the heck isn't __typecheck() in typecheck.h, to be included by
> minmax.h.

And why would you want to use __typecheck() anyway?
It pretty much isn't the test you are looking for.
If you are trying to explicitly avoid converting negative value
to large positive unsigned ones then you want something like:
	is_signed_type(typeof(a)) == is_signed_type(typeof(b))
but it isn't even that simple :-)

	David

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